linux-kernel - Re: [PATCH v2] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning

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Date:   Thu, 3 Feb 2022 15:11:57 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Waiman Long <longman@...hat.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Justin Forbes <jforbes@...hat.com>,
        Rafael Aquini <aquini@...hat.com>
Subject: Re: [PATCH v2] mm/sparsemem: Fix 'mem_section' will never be NULL
 gcc 12 warning

On Tue,  1 Feb 2022 19:35:50 -0500 Waiman Long <longman@...hat.com> wrote:

> The gcc 12 compiler reports a "'mem_section' will never be NULL"
> warning on the following code:
> 
>     static inline struct mem_section *__nr_to_section(unsigned long nr)
>     {
>     #ifdef CONFIG_SPARSEMEM_EXTREME
>         if (!mem_section)
>                 return NULL;
>     #endif
>         if (!mem_section[SECTION_NR_TO_ROOT(nr)])
>                 return NULL;
>        :
> 
> It happens with both CONFIG_SPARSEMEM_EXTREME on and off. The mem_section
> definition is
> 
>     #ifdef CONFIG_SPARSEMEM_EXTREME
>     extern struct mem_section **mem_section;
>     #else
>     extern struct mem_section mem_section[NR_SECTION_ROOTS][SECTIONS_PER_ROOT];
>     #endif
> 
> In the CONFIG_SPARSEMEM_EXTREME case, mem_section obviously cannot
> be NULL, but *mem_section can be if memory hasn't been allocated for
> the dynamic mem_section[] array yet. In the !CONFIG_SPARSEMEM_EXTREME
> case, mem_section is a static 2-dimensional array and so the check
> "!mem_section[SECTION_NR_TO_ROOT(nr)]" doesn't make sense.
> 
> Fix this warning by checking for "!*mem_section" instead of
> "!mem_section" and moving the "!mem_section[SECTION_NR_TO_ROOT(nr)]"
> check up inside the CONFIG_SPARSEMEM_EXTREME block.
> 
> ...
>
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1390,11 +1390,9 @@ static inline unsigned long *section_to_usemap(struct mem_section *ms)
>  static inline struct mem_section *__nr_to_section(unsigned long nr)
>  {
>  #ifdef CONFIG_SPARSEMEM_EXTREME
> -	if (!mem_section)
> +	if (!*mem_section || !mem_section[SECTION_NR_TO_ROOT(nr)])
>  		return NULL;
>  #endif
> -	if (!mem_section[SECTION_NR_TO_ROOT(nr)])
> -		return NULL;
>  	return &mem_section[SECTION_NR_TO_ROOT(nr)][nr & SECTION_ROOT_MASK];
>  }
>  extern size_t mem_section_usage_size(void);

What does the v1->v2 change do?

--- a/include/linux/mmzone.h~mm-sparsemem-fix-mem_section-will-never-be-null-gcc-12-warning-v2
+++ a/include/linux/mmzone.h
@@ -1390,11 +1390,9 @@ static inline unsigned long *section_to_
 static inline struct mem_section *__nr_to_section(unsigned long nr)
 {
 #ifdef CONFIG_SPARSEMEM_EXTREME
-	if (!*mem_section)
+	if (!*mem_section || !mem_section[SECTION_NR_TO_ROOT(nr)])
 		return NULL;
 #endif
-	if (!mem_section[SECTION_NR_TO_ROOT(nr)])
-		return NULL;
 	return &mem_section[SECTION_NR_TO_ROOT(nr)][nr & SECTION_ROOT_MASK];
 }
 extern size_t mem_section_usage_size(void);
_